updating ea settings via file, dll, or by server

If all experts are running in the same Terminal then you can use the built-in file functions. If they are on different Terminals, then u need to use file-functions that can access an external shared folder (unless u want to use some kind of external application to synchronize between the Terminal's '\experts\files' folders). A good starting point is this article -> 'File Operations via WinAPI', or u can use one of the user written libraries -> https://www.mql5.com/en/forum/118999, https://www.mql5.com/en/code/8577. As to the specifics of the implementation, that's too broad of a question - ask specific questions.

p.s. If all experts are in the same Terminal, then by far the simplest solution is via GV's.

ES
In addition to Gordons useful tips, you could read settings from a file on a web server
That would allow you to scale out to multiple EAs, multiple instances of MT, multiple VPS, etc without changing method
See https://www.mql5.com/en/forum/105533
and especially this https://www.mql5.com/en/code/9334
Good Luck
-BB-